Help to building cost function

PennyLane Help
7 
 def states(self, nq=None)->list:
        if not nq:
            nq = self.num_qubits
        st = []
        for i in range(2 ** nq):
            formStr = '0' + repr(nq) + 'b'
            st.append(format(i, formStr))
        return st

 def ansatz(self, params, wires, state_idx=0)-> None:
        state = self.states(nq =self.num_qubits)
        
        for ind_qb, qb in enumerate(list(state[state_idx])):
            if qb =='1':
                qml.PauliX(wires=wires[ind_qb])

then cost total cost is defined